A normal heartbeat involves generation of an electrical impulse and propagation of the electrical impulse across the heart, which causes each chamber of the heart to appropriately contract. Sometimes aberrant conductive pathways develop in heart tissues, and these disrupt the normal path of the electrical impulse. For example, anatomical obstacles or conduction blocks in heart tissue can disrupt the normal propagation of an impulse by causing the impulse to degenerate into several circular wavelets that circulate about the obstacles, thus disrupting normal activation within the heart tissue and chambers. Slow conduction zones in animal and human hearts constrained by anatomical or conduction blocks are believed to exist. Such a zone is a localized region of the heart tissue which propagates an impulse at a slower speed than normal heart tissue thus sometimes resulting in errant, circular propagation patterns or reentrant pathways. Reentrant pathways provide the substrates for the re-excitation of a region of cardiac tissue by an excitatory wavefront, reentry may continue for one or more cycles and sometimes result in tachycardia. Reentrant ventricular tachycardia (VT) is an abnormally rapid ventricular rhythm with aberrant ventricular excitation (wide QRS complexes), usually in excess of 150 per minute, which is generated within the ventricle of the heart as a result of a reentrant pathway.
To treat VT, it is desirable first to determine the physical location of the aberrant pathways. Once located, the heart tissue in the pathway can be destroyed in a process termed ablation by heat, chemicals, and/or other means. Heat can be generated in the targeted tissue using, for example, RF, microwave, ultrasonic, or lasers to effect the ablation lesion. Ablation can remove the aberrant conductive pathway, restoring normal myocardial contraction. More specifically, to treat VT, the slow conduction zone must be located and destroyed or partially destroyed, with the goal of eliminating the slow conduction zone's ability to conduct electrical impulses.
Existing ablation techniques for treating VT tend to focus on creating large lesions to ensure a high cure rate. In fact, companies that produce ablation apparatus compete to produce ablation electrodes that create larger volume lesions. For example, a chilled ablation tip electrode produces a larger lesion than a conventionally-sized ablation tip. Other technologies being pursued to produce larger ablation lesions include using microwave catheters, laser catheters, and chemical ablation.
Tissue ablation for treating VT, however, involves a trade-off. The trade-off is between the need to produce a large enough lesion to ensure a high cure rate and the need to limit the destruction of other viable heart tissue. In certain situations, large surface area lesions that are not very deep may be preferred. For example, if the slow conduction zone is located near the endocardial surface of the heart, creation of a shallow lesion having a large surface area is preferred over a deep lesion. An ablation method for creating a shallow lesion by itself, however, is not completely helpful if it fails to teach how one determines when only a shallow lesion is necessary. Without this determination, one would have to create lesions as part of a trial and error approach. The trial and error method, however, is harmful to a patient since many lesions may be placed at incorrect locations, unnecessarily destroying viable heart tissue. Therefore, what would be helpful, and what is lacking, is a way of determining that only a shallow region is required.